The present invention relates to a method for labeling metal oxides and to a labeled substrate. More particularly, the invention relates to a method for labeling deposits on turbine engine parts to determine effectiveness of cleaning/stripping solutions and procedures.
A typical gas turbine engine includes a compressor, a combustor and a turbine. Gases flow axially through the engine. Compressed gases emerging from the compressor are mixed with fuel and burned in the combustor. Hot products of combustion emerge from the combustor at high pressure and enter the turbine where thrust is produced to turn the turbine, which in turn drives the compressor.
The compressor and the turbine include alternating rows of rotating and stationary coated airfoils. High temperature combustion gases degrade the coatings through either hot corrosion or oxidation. Gases that circulate through the airfoils, particularly during operation on the ground, also include contaminants such as dirt that has been ingested by the engine. Accumulation of dirt can impede effective cooling and if melted, can infiltrate and destroy thermal barrier coatings (TBCs).
The dirt typically comprises mixtures of Ca, Mg, Al, Si, Ni and Fe carbonates and oxides such as multi-elemental spinels (AB2O4). The dirt oxides combine to form particularly deleterious calcium-magnesium-aluminum-silicon-oxide systems (Caxe2x80x94Mgxe2x80x94Alxe2x80x94Sixe2x80x94O), referred to as CMAS. These contaminants can be in a molten state and can infiltrate pores and openings in engine parts to cause crack formation and part failure. Also, a low melting point eutectic Ca3Mg4Al2Si9O30, similar in composition to diopside, can form from silicate-containing dirts at engine temperatures near 1200xc2x0 C. The eutectic can wet and infiltrate coatings. Other contaminants include iron and nickel oxides, sodium vanadates, sodium sulfates, sodium phosphates and the like.
Other turbine engine part contaminants include thermally grown oxides (TGOs). High temperature engine operation can result in TGOs on coatings, which can unintentionally protect an underlying metal coating during chemical cleaning/stripping. For example, alumina scales that form on a diffusion, aluminide coating or on a metallic MCrAlY coating impede chemical attack during stripping, thus leading to incomplete coating removal or excessive base metal attack, both of which can lead to additional rework or part destruction. TGO systems include Cr2O3 and CoxCryO spinels, which form on cobalt-based superalloys such as FSX414. These systems can impede subsequent weld and braze repair processes.
Consequently, airfoils and other turbine engine parts are periodically cleaned and repaired by stripping degraded coatings, mechanically repairing the part and recoating the part substrate. Removal of the degraded coating can be accomplished through one or more stripping or cleaning immersions. Repair of turbine engine parts can also involve cleaning cracks, crevices and substrates to remove CMAS and other oxides, organic and inorganic impurities and dirt prior to alloy filling and brazing. A typical repair process consists sequentially of a dirt clean, coating strip, then a fluoride ion cleaning (FIC) or etching prior to weld/braze repair.
In another repair method, engine-run TBC coatings are stripped from aircraft blades by a caustic autoclave process. Chemical attack occurs at the interfacial alumina TGO, which develops between the bond-coat and the overlying TBC. However, the autoclave process often leaves residual alumina and TBC on the substrate. Residual alumina and TBC can impede subsequent re-coating repairs.
The effectiveness of a chemical cleaning/stripping system can be evaluated by an oxide detection procedure. This procedure involves a non-destructive (NDE) point probe technique using a surface capacitance probe, a surface resistance probe or a thermoelectric probe. However, these procedures are unreliable. The procedures detect gross regions of TGO but not small residual regions. Additionally, there is no NDE method to determine residual alumina or TBC after a TBC removal procedure.
There is a need for a reliable method to determine effectiveness of a cleaning/or stripping or TBC removal procedure. There is a need for a quick method to detect residual alumina and TBC material after caustic autoclave stripping of TBC to reduce rework.
The invention relates to a method to determine effectiveness of a cleaning/stripping system. The invention provides a quick method to detect residual metal oxides including alumina and TBC material. According to the method, a fluorescent probe is applied to bind to a metal oxide on a substrate and the substrate is exposed to an ultraviolet light to identify the metal oxide.
In an embodiment, a chemical cleaning/stripping solution is selected by combinatorial high throughput screening (CHTS). In the method, an array of regions is defined on a substrate, a candidate cleaning/stripping solution is deposited onto the regions to effect cleaning/stripping of the regions; a fluorescent probe is applied to a residual metal oxide left on the substrate after the cleaning/stripping; the substrate is exposed to an ultraviolet light to identify the metal oxide and a product of the cleaning/stripping is evaluated according to the identified metal oxide.
In another embodiment, an activated metal substrate composition is provided that comprises a metal substrate with a contaminant metal oxide coating and a fluorescent activator bound to the substrate by reaction with the metal oxide coating.